A typical turbomachine, such as a gas turbine engine, has an annular axially extending flow path for conducting working fluid sequentially through a compressor section, a combustion section, and a turbine section. The compressor section includes a plurality of rotating airfoils, referred to as compressor blades, which add energy to the working fluid. Upon exiting the compressor section, the working fluid enters the combustion section. Fuel is mixed with the compressed working fluid and the mixture is ignited to thereby add more energy to the working fluid. The resulting products of combustion are then expanded within the turbine section. The turbine section includes another plurality of rotating airfoils, referred to as turbine blades, which extract energy from the expanding fluid. A portion of this extracted energy is transferred back to the compressor section via a rotor shaft interconnecting the compressor section and turbine section. The remainder of the energy extracted may be used for other functions.
Efficient transfer of energy between the working fluid and the airfoils of the compressor and turbine sections is dependant upon many parameters. One of these is the orientation of the rotating airfoil relative to the flow direction of the working fluid. For this reason, a stage of non-rotating airfoils, referred to as vanes, are typically located upstream of each stage of rotor blades. The vanes properly orient the flow for engagement with the blades. Another parameter is the size and shape of the airfoils, both blades and vanes. Typically the airfoils are as thin in the lateral dimension as possible to reduce the weight of the airfoil without affecting the airfoil shape. A limitation to the lateral dimension is the location within the airfoil of cooling passages. Cooling passages are needed to maintain the temperature of the airfoil within acceptable limits.
The amount of energy produced by the combustion process is proportional to the temperature of the resulting products of combustion. For a given fuel and oxidant, increasing the energy of combustion results in a corresponding increase in the temperature of the products of combustion. The allowable temperature of the turbine structure exposed to the hot working fluid, however, typically provides a temperature limit for the combustion process. This temperature limit governs the energy generated by the combustion process.
The allowable temperature within the turbine section is dependant upon material characteristics and stress levels. Turbine materials are maintained below their melting temperature. The allowable temperature of a given component is further limited by the stress level of the component. Allowable stress level is adversely affected by temperature. Therefore, components subject to high stress must be maintained at temperatures well below their melting temperature. This is especially significant for turbine components subject to rotational forces, such as turbine airfoils.
One method to prevent overheating of turbine components is to cool the turbine section using cooling fluid drawn from the compressor section. Typically this is fluid which bypasses the combustion process and is thereby at a much lower temperature than the working fluid in the turbine section. The cooling fluid is flowed through and around various structure within the turbine section. A portion of the cooling fluid is flowed through the turbine airfoils, which have internal passageways for the passage of cooling fluid. As the cooling fluid passes through these passageways, heat is transferred from the turbine airfoil surfaces to the cooling fluid. The passageways include a variety of mechanisms, such as trip strips and pedestals, to maximize heat transfer between the cooling fluid and the turbine airfoil. The cooling fluid exits into the flow path through cooling holes distributed about the airfoil section of the turbine airfoil.
A detrimental result of using compressor fluid to cool the turbine section is a lower overall efficiency for the gas turbine engine. Since a portion of the compressed fluid is bypassing various stages of the turbine section, there is no transfer of useful energy from the compressor fluid to the bypassed turbine stages. The loss of efficiency is balanced against the higher combustion temperatures which can be achieved by cooling with compressor fluid. This balancing emphasizes the need to efficiently utilize the cooling fluid drawn from the compressor section. Efficient utilization of cooling fluid requires getting maximum heat transfer from a minimal amount of cooling fluid.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop means to efficiently cool turbine airfoils to maximize the overall efficiency of a turbomachine.